The invention relates to a directional filter for luminescent screen used for displaying information. It relates more generally to any information display system using one or more of these screens.
The observation of luminescent screens--cathode-ray tubes (TRC) for example--and the information displayed thereon may be rendered difficult by the lighting of the surroundings: surrounding lighting which is too intense impairs the proper reading of the luminous traces appearing on the screens; this is the case particularly in the cockpit of aircraft lit up by the sun or by the light diffused from the clouds. The part of the surrounding light reflected by the screen is superimposed on the luminous trace to be read and impairs suitable reading of the information. The contrast with which this trace appears is insufficient to enable it to be properly perceived. The difficulty remains practically the same with polychrome screens, presenting traces of several different colors.
Different solutions to this difficulty have been practised:
that of the neutral filter in particular. A filtering element having a certain transmission coefficient T is placed in front of the screen; the coefficient T is the fraction of the transmitted light corresponding to a given incident light, whatever the direction in which it reaches the filter.
The part of the surrounding light reflected by the screen undergoes twice the reduction by the coefficient T before reaching the eye of the observer, once to reach the screen, and a second time when it is reflected thereby. It is then attenuated in the ratio T.sup.2, whereas on the contrary the light coming from the trace to be observed only undergoes this reduction once. The contrast is then improved in the proportion of 1/T. If T=10%, it can be seen that this improvement is that of a factor 10, and
that of so-called selective or passband filters absorbing the whole of the visible light with the exception of the light of the trace.
Another type of solution to this difficulty consists in directional filters, valid not only for traces of several colors but also for all traces of the same color.
A directional filter is an optical system whose transmission depends on the angle of incidence. It is generally formed from a flat plate having a multitude of holes of small diameter bored therein. An indicatrix, or curve giving the transmission coefficient with respect to this angle is made to correspond therewith; this curve presents a maximum in the sighting axis and values which progressively decrease away from this axis, to become practically zero beyond a certain value of the angle, called angle of extinction, i.e. outside a so-called extinction cone.
The axial transmission coefficient of such filters may reach several tenths and the reflection coefficient 0.3% only outside the extinction cone.
The invention relates to one of these directional filters.
But, whereas in the prior art these filters were generally constructed by stacking plates, for example photographic negatives having a honeycomb pattern thereon, metal plates having circular holes bored therethrough in a given arrangement--the invention provides, on the contrary, for this filter to be formed from a single plate or a matrix enclosed between supports under conditions which will be set forth.
In general, the superimposed-plate solutions give in fact too high a coefficient of reflection, in the case of photo negatives in particular; because of the dimensions of the openings made, they also give too large an angle of extinction.
The filter of the invention must allow observation in luminous surroundings with lighting going up to 100,000 lux.